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Carbon and nitrogen isotope disturbances and an end-Norian (Late Triassic) extinction event

¹ Planetary and Space Sciences Research Institute, Open University, Milton Keynes MK7 6AA, UK
² School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
³ Planetary and Space Sciences Research Institute, Open University, Milton Keynes MK7 6AA, UK
⁴ School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
⁵ Geological Survey of Canada, 3303-33rd Street Northwest, Calgary, Alberta T2l 2A7, Canada

Major perturbations of organic carbon and nitrogen isotope ratios from a Norian-Rhaetian (Late Triassic) boundary section in British Columbia coincide with an extinction of the dominant, deep-water invertebrate fauna of the Late Triassic (monotids and most ammonoids). The carbon isotope excursion is attributed to the development of widespread oceanic stagnation that favored organic-rich shale deposition. The coincident nitrogen isotope excursion suggests that progressively more nitrate-limited productivity forced a change to nitrogen-fixing cyanobacteria populations as ocean stagnation created nutrient-starved conditions. The biotic crisis and geochemical events of the Norian-Rhaetian boundary predate the latest Rhaetian (end-Triassic) mass extinction. Thus, the Late Triassic interval was marked by more than one extinction event.

Key Words: Triassic • Norian • Rhaetian • extinction • carbon • nitrogen • isotopes • anoxia

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